Re: RFC: Explicit move preference hints

[Vladimir Makarov <vmakarov at redhat dot com>]

On 08/22/2017 06:48 AM, Wilco Dijkstra wrote:
> Hi,
>
> The register allocator inserts move preferences when an instruction has
> one or more dead sources in add_insn_allocno_copies. If an instruction
> doesn't have a matching constraint (eg. "0"), then any dead source is treated
> as a copy with all destination registers with a low priority. In reality what
> appears to happen is that the first dead source is treated as a copy. This
> leads to non-intuitive allocations in eg. 4-register FMAs. Here you'd
> prefer to have the accumulator and destination to use the same register
> when possible: so fmadd  d2, d0, d5, d2 instead of fmadd  d0, d0, d5, d2.
>
> Would it be reasonable to add an explicit move preference feature?
> For example use "r#0" to indicate in ira_get_dup_out_num that an operand
> prefers to be the same as the destination if possible. This would use a lower
> move priority than "0" to avoid constraining the allocation but a bit higher than
> a simple dead source (eg. freq / 4 or freq / 2).
Wilco, sorry for a delay with the answer.  I needed some time to think 
about it.

Thank you for writing about the problem.

As I correctly understand, you just want an intuitive allocation. The 
current allocation performance has the same quality as the intuitive one.

In this case it would be hard for me to approve such change because it 
gives no performance improvement but complicates machine description 
definition which is already too complicated.

I think if it is important for some cases may be it is possible to find 
an alternative solution without introducing a new hint.  Segher proposed 
some solutions.  They might work.  If they don't work  we could try to 
change heuristics in LRA, for example, to favor copy destination with 
the first operand to improve some regularity for human eyes.